Lumber and Siding - Replacing Exterior Trim

I recently replaced a window and am replacing the surrounding trim (exterior of home). I used untreated pine which is not primed (couldn't find 11.25" primed pine). My question is how long can I leave the untreated wood exposed to the elements? I likely won't be able to prime/paint it until next weekend. Also, the original trim had 1/4" cedar plywood spacers between the sheathing and the trim. Is there a reason why the trim wasn't nailed directly against the sheathing? What is the gap for? Thanks.

It shouldn't hurt the pine to go unpainted for several weeks. I'd recomend using an oil base exterior wood primer as it will seal the wood better than a latex primer. Latex is fine for the top coat. I don't know about the spacers.

The spacers are likely meant to pad a thin material out so that it appears thicker. In this case, it was probably so that the front edge of the trim would not be recessed behind the front edge of the window frame.

Makes sense XSleeper. Thanks